Personally, I'd take the D2. I've used both the beseler 23c and the omega D2. As far as I know, there's no bellows between the condenser and the negative stage. If it looks like there's loose space there it's probably because you need to put a negative carrier in there. When I was looking for an enlarger I'd been searching for a D2, but in frustration I ended up getting an Omega B-8, which is more or less a slightly smaller version of the latter. It's probably easier to get a color head for the omega than to convert the beseler to 4x5. The D2 with a color head would be a wonderful enlarger, imo. I so very nearly obtained such a machine but the amount of magical green paper I had at the time held me back.